Photolithographic mask and methods for producing a structure and of exposing a wafer in a projection apparatus

ABSTRACT

Auxiliary openings are assigned to openings on a mask to be transferred to a wafer. These auxiliary openings have a phase-shifting property, preferably between 160° and 200° with respect to the openings, and a cross section lying below the limiting dimension for the printing of the projection apparatus, so that the auxiliary openings themselves are not printed onto the wafer. However, the auxiliary openings do enhance the contrast of the aerial image, in particular of an associated, isolated or semi-isolated opening on the wafer. The auxiliary openings may have a distance from the opening that lies above the resolution limit of the projection apparatus but that is less than the coherence length of the light used for the projection. A phase-related utilization of the optical proximity effect results, which can produce elliptical structures from square openings on the mask when the auxiliary openings are disposed in the preferential direction.

CROSS-REFERENCE TO RELATED APPLICATION

[0001] This is a continuation of International applicationPCT/DE01/04263, filed Nov. 14, 2001, which designated the United Statesand which was not published in English.

BACKGROUND OF THE INVENTION FIELD OF THE INVENTION

[0002] The present invention relates to photolithographic masks, inparticular, to photolithographic masks for structuringradiation-sensitive resist layers on semiconductor substrates in orderto produce highly integrated semiconductor components.

[0003] Projection apparatuses operating with optical light normally inthe ultraviolet length range are in widespread use as wafer steppers orscanners, as they are known, in the production of semiconductorproducts. By using regions that are transparent or opaque to radiation,that is to say opaque, defined structures, which are typically formed ona Quartz plate as a mask, are transilluminated with coherent,monochromatic light and, via a lens system, and are brought to form animage on a wafer coated with a light-sensitive varnish.

[0004] As a rule, sets of the masks provided by the structured quartzplates are used in order to build up the layers or the levels on a wafergradually. In the production of integrated circuits on a wafer, thephysical resolution limits for the optical projection limit the possiblestructure sizes of individual planes. One typical example is specificplanes for the production of memory products (DRAM). For the resolutionof the image, it is true that:

b _(min) =k _(l) *λ/NA  (1)

[0005] Here, b_(min) is the minimum structure line width, μ is thewavelength of the monochromatic light, and NA is the numerical apertureof the imaging lens system. The coefficient k_(l), includes, firstly,techniques utilizing the diffraction characteristics of the light, suchas oblique-light illumination, phase-shifting structures or OPCstructures (optical proximity correction), as they are known, andsecondly, this factor also accounts for problems such as lens faultsleading to aberration (coma, chromatism).

[0006] The minimum structure line width on the mask, which is actuallyproduced after the production of the resist structure, is oftencalculated to be larger than from equation (1), for several reasons.Firstly, the resist layer has a finite thickness, so that the image iseasily smeared; furthermore, the developer acts isotropically, so thatduring the development of the resist layer, the resist is also removedin the lateral direction. The minimum structure line width on the maskthat is needed for the production of a resist layer structure on asemiconductor substrate therefore depends on many parameters and isdetermined individually for each structuring process.

[0007] Various effects can contribute to impairing the fidelity of themask. Firstly, the finite resist contrast λ, which is a measure of theresist-remove gradient, causes originally angular mask structures to berounded. Furthermore, interference effects, diffraction effects andscattered light, which arise at structure elements of the mask, theresist layer, and/or the pre-structured substrate surface, lead to theeffective exposure dose not being homogeneous in the resist layer areas.

[0008]FIG. 1 illustrates the aforementioned difficulties on aconventional lithographic mask, which has a substrate 51 transparent toradiation, for example made of glass, and a layer 52 opaque toradiation, for example made of chromium. In this case, the openings 1 inthe layer 52 opaque to radiation correspond to the structure that is tobe transferred to the photoresist layer on the wafer in an appropriatemasking step. During an exposure, radiation—for example ultravioletlight—passes through the openings 1 in the layer 52 opaque to radiationand, because of interference effects, leads to the distributionillustrated of the electrical field E in the photoresist layer on thewafer.

[0009] Because of interference effects, undesired exposure in thephotoresist layer occurs between the openings 1 and 4, an actual darkregion on the mask. Since the exposure intensity is proportional to thesquare of the field strength, the field strength distribution shown inFIG. 1 leads to a corresponding intensity distribution I in thephotoresist layer.

[0010] In order to avoid these difficulties and to improve the structureresolution, use is therefore also increasingly being made of“alternating phase masks” instead of the “dark field masks” previouslydescribed. In this case, each second opening 4 in the layer 52 opaque toradiation has a phase variation applied to it, for example by etchingthe glass substrate 51, such that a phase difference between adjacentopenings 1 and 4 is achieved. As a rule, 180° is set as the phasedifference. By applying this technique, in the case of highly periodic,grid-like structures, an increase of up to a factor 2 can be achieved inthe structure resolution, as compared with the conventional technique.

[0011]FIG. 2 illustrates the situation that results. Because of the 180°phase difference between adjacent openings 1, there is now destructiveinterference between the radiation that passes through the left-handopening 1 and the radiation that passes through the right-hand opening4. The field distribution E in the photoresist layer therefore now has azero point between the two openings 1, which therefore also leads to adistinctly lower intensity I between the two openings 1. In this way,the contrast of the exposure is increased considerably.

[0012] However, this positive effect occurs only for structures opaqueto radiation which, on two opposite sides, have an opening with a phasedifference. Since the patterns formed by the openings correspond to thestructures that are to be imaged or transferred into the photoresistlayer, it is, however, possible for situations to occur in whichopenings with only one adjacent opening or completely isolated openingsoccur. In this case, it is possible for such a semi-isolated orcompletely isolated opening not to be imaged completely into the resistlayer. Until now, attempts have been made to ensure transfer into thephotoresist layer by widening the corresponding openings, at least underoptimum lithographic conditions (optimum focus, nominal exposure).However, the lithographic process window is then so small that thecorresponding structures often lead to failure of the component in thefabrication process. Accordingly, this technique is only used in rarecases in practice, which results in critical structures in the layouthaving to be forbidden, but this results in a drastic restriction in theuse of alternating phase masks.

SUMMARY OF THE INVENTION

[0013] It is accordingly an object of the invention to provide aphotolithographic mask and methods for producing a structure and ofexposing a wafer in a projection apparatus that overcome thehereinafore-mentioned disadvantages of the heretofore-known devices ofthis general type and that provide a photolithographic mask that reducesthe above-described problems or avoids them entirely and which, inparticular, is also capable of transferring isolated structures into thephotoresist layer with a high resolution.

[0014] With the foregoing and other objects in view, there is provided,in accordance with the invention, a photolitographic mask for exposingradiation-sensitive resist layers on semiconductor substrates isprovided, the mask

[0015] a) having at least one layer opaque to radiation on a transparentcarrier material,

[0016] b) at least one opening being provided in the layer opaque toradiation, the at least one opening being formed in such a way that thepattern formed by the at least one opening is transferred into theresist layer during an exposure, and

[0017] c) at least one auxiliary opening being provided in the layeropaque to radiation,

[0018] the at least one auxiliary opening being formed in such a waythat the pattern formed by the at least one auxiliary opening is nottransferred into the resist layer during an exposure, and

[0019] as the radiation passes through the at least one auxiliaryopening, a phase difference being produced with respect to the radiationthrough an adjacent opening or an adjacent auxiliary opening.

[0020] Throughout the application, the term “opening” (as distinguishedfrom “auxiliary opening”) is synonymous with the term “primary opening”.

[0021] The photolithographic mask according to the invention providesfor the application of auxiliary openings in addition to the actualopenings determining the structure. In the case of an associationaccording to the invention between opening and auxiliary opening, theopening will also be referred to as a mother structure below. In thiscase, the auxiliary openings have a phase variation matched to theadjacent openings. Within the context of the present application, twoopenings are referred to as adjacent when the radiation that passesthrough the two openings gives rise to substantial interference effects.With the auxiliary openings, it is possible to achieve a considerableimprovement in the aerial image contrast, especially outside the imageplane, and therefore to ensure a considerably enhanced depth of focus.Since the auxiliary openings are formed in such a way that they are notimaged in the photoresist, they may also be referred to assub-resolution phase assist structure (SPAS).

[0022] The use of these auxiliary openings leads to a considerableenlargement in the process window, in particular in the case of isolatedor semi-isolated structures, and also to a reduction in the differencein the line width relating to closely packed structures. It furtherpermits the exposure of circuit-typical layouts in a single exposure,instead of with a double exposure—as in the prior art—and therefore withtwice the productivity. The openings often have a rectangular shape. Theopenings generally are considerably longer than they are wide. In thiscase, the auxiliary openings are preferably formed parallel to theactual openings.

[0023] According to a preferred embodiment, the width of the auxiliaryopenings is selected to be less than 0.3 λ/NA. In this case, λdesignates the wavelength with which exposure is carried out, and NA thenumerical aperture.

[0024] Furthermore, it is preferred for at least one auxiliary openingto be provided in relation to a semi-isolated opening. Within thecontext of the present application, an opening is referred to assemi-isolated if it has an adjacent opening only in one direction. Inthe case of a semi-isolated opening, at least one auxiliary opening, istherefore provided, which replaces the “missing” adjacent opening in theopposite direction.

[0025] Furthermore, it is preferable if at least two auxiliary openingsare provided in relation to an isolated opening. Within the context ofthe present application, an opening is referred to as isolated if itdoes not have any adjacent opening. In the case of an isolated opening,therefore, at least two auxiliary openings are provided, which replacethe “missing” adjacent openings.

[0026] According to a preferred embodiment, as the radiation passesthrough adjacent openings, a phase difference of 180° in relation toeach other is produced in each case. Furthermore, it is preferred if, asthe radiation passes through an auxiliary opening and its adjacentopening or its adjacent auxiliary opening, a phase difference of 180° isproduced.

[0027] Furthermore, it is preferred if the openings and/or the auxiliaryopenings form a grid-like pattern. In this case, it is preferred if theauxiliary openings are disposed at a distance from the adjacent openingor auxiliary opening which amounts to approximately 0.3 to 0.7 times theperiod of the grid.

[0028] In a particularly advantageous development, the at least oneopening has a rectangular structure, while the at least one auxiliaryopening

[0029] has a cross section below a limiting dimension, which describesthe minimum structure extent on the mask necessary to form a structureon the semiconductor substrate,

[0030] is in each case disposed at a distance from the at least oneopening, the distance

[0031] a) being greater than the resolution limit of the projectionapparatus used for the exposure, based on the wafer scale, and

[0032] b) being less than the coherence length of the light used in theprojection apparatus,

[0033] as the radiation passes through, a phase difference is producedwith respect to the radiation through an adjacent opening.

[0034] In order to escape from the high requirements in theabovementioned critical planes, distressing of the overlay, that is tosay the positional accuracy between the planes, is often desirable.Attempts have been made to expand structures on the semiconductorsubstrate or wafer in that coordinate direction in which, possibly,there are just not the high requirements on the minimum structure width.This applies generally when, in precisely this coordinate direction, thedistances to adjacent structures on the substrate are firstlysufficiently large and secondly the functionality of the circuit is notimpaired.

[0035] For the memory products mentioned at the beginning, the substratecontact-making plane can be mentioned as an example here. In this case,the conventionally square-shaped contact holes on the mask areconfigured as rectangular openings, the longitudinal axis of all therectangles having the same coordinate direction. In a corresponding way,the ellipses with a similar longitudinal extent are to be formed on thewafer.

[0036] In this case, the problem previously occurred that the length towidth ratio present on the mask was not transferred to the wafer in thesame way. Instead, the imaged structures tend to maintain a length towidth ratio lying closer to the value 1. In particular, in the case ofsystems with a very small coefficient k₁, that is to say those withintensive use of the improved lithographic techniques, it is to berecorded that—in order to obtain elliptical structures on the water atall—rectangles with a length to width ratio of more than 1.5 have to bestructured on the mask.

[0037] The use of this technique was therefore limited to mask planelayouts with relatively great distances, for example the contacts in theuncritical coordinate direction. One alternative for circumventing thisproblem included producing an opening with an enlarged longitudinal axison the semiconductor substrate by using a small offset in the mask inthe direction of the desired coordinates in a double exposure of thesemiconductor substrate. In this case, however, the disadvantage arosethat, on the one hand, accurate adjustment of the offset had to becarried out, with a great deal of effort, and also that the productivityof the imaging process for all the planes involved was virtually halved.resolution limit designates that dimension that results by referringback the scale of the resolution limit from the wafer to the mask afterthe projection. If the resolution limit on the wafer is, for example,140 nm, then in the case of a 5:1 projection the result is acorresponding value of 700 nm for the mask. The size statements usedbelow, in particular of the exemplary embodiments, relate to the waferlevel, however, and therefore have to be multiplied for the maskaccording to the invention, depending on the type of projection, by thecorresponding reduction factor, for example 4 or 5.

[0038] As a result of the property that the auxiliary opening has adimension located in the vicinity of the resolution limit, although theopening is imaged on the substrate or wafer as an aerial image of verylow contrast and enhances the contrast of the rectangular structureconsidered here as a mother structure, because of the low imagecontrast, it is itself not imaged as a structure in the resist. In thiscase, in particular, the limiting dimension that describes the structureformation (printing in the following text) can be greater than theresolution limit of the projection apparatus for structures on thewafer, but will generally lie very close to this limit. A reducedintensity resulting from partial absorption of the phase-shifted lightpassing through the auxiliary opening, and also a changed varnishsensitivity of the resist on the wafer additionally determine, interalia, the exact value of this limiting dimension for the printing on thewafer.

[0039] The intensity contributions of phase-shifted light are thereforeimportant in relation to the formation of the mother structure, but arenot sufficient for any direct imaging of the auxiliary opening as suchon the wafer.

[0040] The limiting dimension for the printing may be determineddirectly for a projection apparatus to be used and a varnished layer tobe exposed, that is to say for example a layer thickness or chemicalcomposition, etc, on a wafer with a fixed exposure intensity.

[0041] Since the distance of the auxiliary opening from the opening tobe imaged lies within the coherence length range of the light, theimaging of the opening is advantageously influenced. As a result of theminimum distance, given by the invention, in accordance with theresolution limit of the projection system, this influence does not,however, lead to a direct enlargement of the area of the opening in thedirection of the location of the auxiliary opening, as would be thecase, for example, in the optical proximity effect with equal-phaselight regions, that is to say transparent or semi-transparent regions.

[0042] As a result of the shift in the phase of the light as it passesthrough the auxiliary openings, and the proximity effect caused thereby,it is possible—as has been found by simulation and experiment—for thegeometric shape of the aerial image and consequently that of thevarnished structure produced to be influenced advantageously. By usingthe present invention, auxiliary openings can accordingly be assigned tothe openings to be imaged in such a way that the auxiliary openingsthemselves are not imaged, but influence the projection of the openingsto be imaged in the complementary coordinate direction to such an extentthat a desired expansion of the area of the structure in precisely thisdirection takes place, or thinning of the opening to be imaged takesplace in the mother structure-auxiliary opening coordinate direction.

[0043] The present invention proves to be particularly advantageous inthe production of elliptical structures on the wafer from essentiallysquare structures on the mask. With advantageous utilization of theproximity effect caused by the assisting phase auxiliary openings, theaerial image on the mother structure-auxiliary opening axis isinfluenced in such a way that the dimension of the varnished structureproduced deviates in this coordinate from the coordinate that isvertical with respect thereto. The structure to be imaged will thereforeexpand in its length in this direction, while it remains limited interms of its width in the direction of the auxiliary openings during theimaging onto the wafer. The ellipticity, that is to say the length towidth ratio, can therefore be controlled through the deliberate shapingof the auxiliary openings.

[0044] As a result of this development of the present invention, thereis no restriction to specific original length to width ratios of therectangular structure. On the mask, square and pronounced rectangularmother structures can be influenced in the same way by setting upcorresponding auxiliary openings.

[0045] In particular, the rectangular structures to be imaged can be setup as structures on alternating phase masks. Application of theauxiliary openings according to the invention to half-tone phase masksis likewise conceivable.

[0046] A further advantage of the present invention is the significantenlargement of the process window for the projection. For the parametersdose and focus of the exposure or projection, ranges have to bespecified within which a predefined quality of the image is achieved,the ranges in each case being interdependent. A combination of bothranges for the projection is selected which, firstly, contains the bestpossible imaging, and secondly permits the greatest possible freedom forthe parameters. It is precisely this freedom which is advantageouslyenlarged by the present invention in the case of a given quality limitfor the image. In particular, according to the invention the depth offocus range (focus), for example for imaging a substrate contact-makingplane in memory products, is improved by virtually twice as comparedwith conventional chromium masks.

[0047] In a further refinement of the present invention, a particularlyeffective range was found for the phase difference according to theauxiliary opening with respect to the rectangular structure to beimaged, which is embedded between 160° and 200°.

[0048] In a further refinement, in order to achieve the most homogeneouseffect, for example in one coordinate direction, a length of at leastthe length of the rectangular structure to be imaged is assumed for theauxiliary opening. Since the cross section, for example the width, liesin the range of the resolution limit of the projection apparatus, whichis given by 0.25*λ/NA, in spite of this longitudinal expansion, theauxiliary opening is not printed on the wafer.

[0049] In a further advantageous refinement, a further auxiliary openingis provided mirror-symmetrically with respect to the previous auxiliaryopening about the axis of symmetry of the at least one rectangularstructure. As a result, a symmetrical longitudinal expansion of therectangular structure is also ensured on the wafer—or a square structureas a subset of the rectangular structures.

[0050] A further advantageous refinement of the present inventioninvolves under-structuring the auxiliary opening. For example, for adark field mask, an auxiliary opening can include interrupted elongatedholes. In turn, these intrinsically form rectangles or lines, which areinterrupted from one another at a distance lying underneath theresolution limit. The action of such auxiliary openings is in this caseinsignificantly restricted with respect to the uninterrupted case.

[0051] According to the present invention, setting up the auxiliaryopening relative to the rectangular structure is not restricted toemphasizing one coordinate direction. In order, for example, to enlargeonly the process window, an alternative course is to provide auxiliaryopenings according to the invention on all four sides of a rectangle.For example, a rectangular frame configured as an auxiliary opening—oras a set of auxiliary openings—around the rectangular structure to beimaged also leads to an advantageous enlargement of the process window.

[0052] In a further advantageous refinement, the auxiliary openings thatare associated with the rectangular structures to be imaged form,together with the latter, a regular pattern on the mask. In specificcases of the forming of a grid of rectangular structures provided inthis way—for example in the case of the substrate contact-making planein memory products—the individual auxiliary openings can be combined intheir longitudinal extent to form elongated structures. The crosssection of the auxiliary opening at the same time, according to theinvention, satisfies the criterion of a sub-resolution structure, as itis known. The advantage resides in the fact that, according to theinvention, the effect that produces an elliptical structure is achieved,while the outlay for the mask configuration and the mask itself is keptlow.

[0053] As a result of a further refinement, the rectangular structure tobe imaged is substantially square. In interplay with the configurationof a regular pattern of rectangles, the result here is the significantadvantage that, in order to achieve any desired regular rectangularpattern on the wafer, rectangular structures which are equipped with astill larger length to width ratio no longer have to be provided on themask in any case. Instead, these can be provided as squares on the mask,as a result of which, more interspace is produced on the mask betweenthe structures, in particular in the case of conventional criticaldistances. One example will be presented below.

[0054] In a further refinement, the rectangular structure and the atleast one auxiliary opening are in each case formed with differenttransparency for the light falling through. By using suitable selectionof the transparency of the auxiliary opening, a further enlargement ofthe process window for the projection can advantageously be achieved.Restricted light transmissivity of the auxiliary opening during thefabrication permits the selection of a large cross section, since thelimiting dimension for the printing then likewise rises.

[0055] In a further refinement, the cross section of the auxiliaryopening is chosen to be smaller than the resolution limit of the opticalprojection system itself. The advantage here is that this variabledepends substantially only on the lens system or its opening and can bespecified immediately. Below this limit, of course, no printing can becarried out on the wafer either. The narrow range between minimumnecessary limiting dimension for the printing as upper limit andresolution limit as lower limit, on the other hand also depends on theresist on the wafer or the type of the processes following the exposure,for example the development or etching.

[0056] In addition, the ellipticity can be controlled by adapting thenumerical aperture of the projection system, it being possible for afurther enlargement of the process window likewise to be achieved.

[0057] Other features that are considered as characteristic for theinvention are set forth in the appended claims.

[0058] Although the invention is illustrated and described herein asembodied in a photolithographic mask and methods for producing astructure and of exposing a wafer in a projection apparatus, it isnevertheless not intended to be limited to the details shown, becausevarious modifications and structural changes may be made therein withoutdeparting from the spirit of the invention and within the scope andrange of equivalents of the claims.

[0059] The construction and method of operation of the invention,however, together with additional objects and advantages thereof will bebest understood from the following description of specific embodimentswhen read in connection with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0060]FIG. 1 is a diagrammatic sectional view of a photolithographicmask according to the prior art and resulting electrical field andintensity distributions;

[0061]FIG. 2 is a sectional view of a further photolithographic maskaccording to the prior art and resulting electrical field and intensitydistributions;

[0062]FIG. 3 is a diagrammatic plan view of a photolithographic maskaccording to a first embodiment of the invention;

[0063]FIG. 4 is a sectional view of the photolithographic mask shown inFIG. 3 and taken along line A-A;

[0064]FIG. 5 is a plan view of a second embodiment of aphotolithographic mask;

[0065]FIG. 6 is a sectional view of the photolithographic mask shown inFIG. 5 and taken along line A-A;

[0066]FIG. 7A is a plan view of a regular pattern of contact holes in asubstrate contact-making plane with openings on the mask according tothe prior art;

[0067]FIG. 7B is an enlarged detailed view, in a matched scale, of theexposed ellipses formed from them on the wafer according to the priorart;

[0068]FIG. 8A is a plan view showing a regular pattern of a substratecontact-making plane having a first example of square openings andauxiliary openings shown as hatched liens;

[0069]FIG. 8B is a plan view showing a regular pattern of a substratecontact-making plane having a second example of square openings andauxiliary openings shown as hatched liens;

[0070]FIG. 8C is a partial detailed view of the substrate contact-makingplane including the resolution limit and limiting dimension, which aredrawn to scale in FIG. 8A;

[0071]FIG. 9 is a plan view of a mask with a square opening, with twoassociated auxiliary openings each including four interrupted elongatedholes; and

[0072]FIG. 10 is a plan view of a square rectangular structure accordingto the invention made by the mask shown in FIG. 9.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0073] Referring now to the figures of the drawings in detail and first,particularly to FIG. 3 thereof, there is shown a photolithographic maskaccording to a first embodiment of the present invention. The mask has agroup of five openings 1 and 4 with a width of 0.35*λ/NA (wafer-based).They are grouped as grids with the period 0.7*λ/NA, adjacent openings 1and 4 having a relative phase difference of 180°. Parallel with theouter openings 1, in each case an auxiliary opening 2 with a width of0.27*λ/NA is made at a distance D of 0.7* λ/NA, the phase variationdiffering by 180° from the adjacent opening. The auxiliary openings 2have the effect that the semi-isolated openings 1 are imaged with aconsiderably higher aerial image contrast, and the exposure method has aconsiderably improved lithographic process window. Because of the lowintensity of their aerial image, the auxiliary openings 2 are nottransferred into the resist layer.

[0074] Furthermore, provision is made for the openings 1 supported byauxiliary openings 2 to be given a width adaptation, in particular abroadening, so that under nominal exposure conditions (best focus,nominal exposure), the openings 1 are transferred into the resist layerat the same width as the adjacent, closely packed openings 4. Dependingon the exposure parameters, the openings 1 supported by auxiliaryopenings 2 can be broadened within a range of up to 20%.

[0075] The isolated opening 1 standing out from the central structure issupported on both sides by auxiliary openings 2 with a width of0.27*λ/NA. The three openings form a grid of period 0.7*λ/NA. FIG. 4shows, schematically, a section along the line A-A in FIG. 3.

[0076]FIG. 5 shows a further embodiment of the photolithographic maskaccording to the invention. The embodiment shown in FIG. 5 isdistinguished by the use of two auxiliary openings 2 and 5 for eachsemi-isolated opening 1 and, respectively, four auxiliary openings 2 and5 for each isolated opening 1. In this case, the phase of the additionalauxiliary opening 5 is selected such that, taking account of the opening4 to be supported, it alternates, that is to say the auxiliary openings2 do not have the same phase as the auxiliary openings 5. FIG. 6 shows across section relating to FIG. 5.

[0077] The problem of producing elliptical structures on a wafer, whichare configured in a regular pattern, is illustrated in FIGS. 7A and 7Busing the example of the substrate contact-making plane for producingintegrated components such as memories. The illustration shows theopenings or rectangular structures 1 according to the prior art on themask and, drawn above them and in matched scale, the structures 1′imaged on the wafer as dashed ellipses. In general, in the case of thesubstrate contact-making plane, the openings 1 on the mask areconfigured as a square contact area, so that according to the prior art,circular contact holes are produced as imaged structures 1′ on thewafer. In this example, the situation is such that in the case of thesecritical structures, that is to say standing close to the resolutionlimit 30, contact holes elongated in one dimension are to be produced asimaged structures 1′ , which is carried out here in the vertical Ydirection in FIG. 7B. As FIG. 7B shows, the ratio of the length 12 ofthe rectangular structure 1 to its width 11 must be at least 1.5 times,in order to achieve ellipticity of the imaged structure 1′ at all.

[0078] The dashed ellipses of the images structures 1′ shown in thefigures represent lines of the same intensity on the wafer. In the caseof more intensive or less intensive exposure, however, although the areof the structure 1′ changes, the length 12′ to width 11′ ratio of theimaged structure 1′ does not change substantially with a varyingintensity.

[0079] In order to produce still greater ellipticity of the imagedstructures 1′ on the wafer, however, one quickly runs against limitswhen structuring the openings 1 on the mask, as can be seen in FIG. 7 onthe right hand side. In the case of the longitudinal expansion of theopenings 1 in order to produce unfortunately only moderate ellipticitieson the wafer, the distance 15 of the openings 1 from the respectivelyadjacent openings must be at least greater than the resolution limit 30.This condition disadvantageously sets an upper limit on the verticallongitudinal expansion of the imaged structures 1′ as contact holes onthe wafer.

[0080] An example according to the invention for solving this problem,which is shown in FIG. 8A, is based on square openings 1, whose image onthe wafer, because of seamlessly joined auxiliary openings 2 which havea phase shift by 180°, are imaged as structures 1′ on the wafer whichhave a high ellipticity. The auxiliary openings 2 have a width 21 thatlies in the range of the resolution limit 30, but below the limitingdimension 31 for the printing, of the projection apparatus. The distance9 of the auxiliary opening 2 on the mask from the opening 1, on theother hand, lies above the resolution limit 30, so that thephase-shifting auxiliary opening 2 acts on the square opening 1 as it isimaged onto the wafer, utilizing the proximity effect. For the exemplaryembodiment in FIG. 8A, a projection apparatus with a wavelength of 248nm and a numerical aperture NA=0.63, and a filling factor σ=0.30 wasselected. The square contact holes 1, based on the wafer level have anextent of 230 nm×230 nm. The seamlessly adjoined auxiliary openings 2,constructed as assist columns in this exemplary embodiment, have a width21 of 100 nm.

[0081] As a result of the square formation of the openings 1, which ismade possible by the action of the auxiliary openings 2 according to theinvention, the problems indicated in the explanation relating to FIG. 7can be circumvented. The degrees of freedom for placing and formingstructures on the mask are improved considerably as a result of the useaccording to the invention of auxiliary openings in the production ofstructures 1′ on the wafer. During the formation of the auxiliaryopenings 2 and during the layout of the mask, their effect can becontrolled precisely via their width 21 and via their distance 9 fromthe rectangular structure 1.

[0082] Likewise, a significant enlargement of the process window isachieved. At a given numerical aperture for the example according to theprior art according to FIG. 7, and the example according to theinvention according to FIG. 8A, the depth of focus could be increasedfrom 0.39 μm to 0.59 μm, and a doubling of the dose variation rangecould also be achieved.

[0083] Since operations are carried out in the substrate contact-makingplane shown in the exemplary embodiment with lengths 12 and widths 11 inthe critical resolution range, the mask error enhancement factor, as itis known, conventionally acts in a particularly disadvantageous mannerhere. Therefore, minimum lines or structure width fluctuations aretransferred in a non-linear way from the mask to the wafer. By using theexemplary embodiment according to the invention, the enlargement of theprocess window is therefore accompanied by a particularly effectiveimprovement in the transfer quality and therefore consequently also areduction in the mask error enhancement factor.

[0084] In addition to the larger process window, a decisive advantage ofthe solution according to the invention involves the fact that, even inthe case of a square configuration of the rectangular structures 1 onthe mask, considerable ellipticity is achieved for the resiststructures. The conventional technique also has closer limits withrespect to the ellipticity because, with a smaller period of thecontacts in the longitudinal direction of the contacts, theaforementioned enlargement of the mother contacts runs up against limitswhich are caused by the mask production, such as the minimum land widthswhich can be inspected or the land widths which can be resolved.

[0085]FIG. 8B shows a further exemplary embodiment, according to whichthe phase-shifting auxiliary openings 2 are disposed horizontally inrelation to the contact holes 1. Here, an ellipticity with majorsemi-axes in the horizontal direction is achieved. The variablesused-based on the wafer level—are: length 12 of the contact hole: 230nm; width 11 of the contact hole; 230 nm; length 22 of the auxiliaryopening; 560 nm; cross section 21 of the auxiliary opening: 110 nm:period of the contact holes in the vertical direction: 560 nm. The phasedifference is 180 degrees, the filling factor σ is 0.30 and thenumerical aperture is 0.63, the wavelength is 248 nm.

[0086]FIG. 9 shows a further development of the example according to theinvention shown in FIG. 8A. The detail shows the square-shapedrectangular structure 1, which is surrounded on the left and right bytwo auxiliary openings 2. The auxiliary openings 2 include interruptedelongated holes 2 a, 2 b, 2 c, 2 d. The distance of the elongated holesfrom one another is smaller than the distance of the elongated holesfrom the rectangular structure 1. In, particular, their distance fromone another is less than the resolution limit 30, so that their actionon the rectangular structure 1 is substantially the same as that of acontinuous auxiliary opening 2, but the interruption of the elongatedholes 2 a-2 d leading to a slight reduction in the area of the auxiliaryopening 2, as a result of which the action of the proximity effectcaused by the phase structure is correspondingly also reduced slightly.The length 22 of the auxiliary opening 2 remains untouched by theinterruption of the elongated holes 2 a-2 d.

[0087] A further configuration according to the invention of fourauxiliary openings 2 around a rectangular structure 1 is shown in FIG.10. In this exemplary embodiment, the aim is not the production of anelliptical structure, rather merely the enlargement of the processwindow for the imaging of the rectangular structure 1 onto the wafer isutilized. The rectangular structure 1 shown in FIG. 10 is notnecessarily square, according to the present invention. It is furtherpossible even for further auxiliary openings 2 disposed downstream ofand behind the auxiliary openings of FIG. 10 to be formed, which arestill located within the range of the coherence length 40 shown in FIG.10. However, it is also necessary here to satisfy the further conditionsthat the overall width 21 of the auxiliary opening 2 does not exceed thelimiting dimension 31, so that no imaging of the auxiliary openings 2 onthe wafer can take place.

We claim:
 1. A photolithographic mask for exposing a radiation-sensitive resist layer on a semiconductor substrate, comprising: a transparent carrier material; and a layer opaque to radiation disposed on said transparent carrier material and having formed therein a primary opening, a first auxiliary opening adjacent said primary opening, and a second auxiliary opening adjacent said first auxiliary opening; said primary opening forming a first pattern to be transferred to the resist layer during an exposure; said first auxiliary opening forming a second pattern not to be transferred to the resist layer during the exposure; and said first auxiliary opening producing a phase difference with respect to the radiation through one of said primary opening and said second auxiliary opening.
 2. The photolithographic mask according to claim 1, wherein: said layer has a further primary opening adjacent said primary opening; and said primary opening producing a relative phase difference with respect to the radiation through said further primary opening as the radiation passes through said primary opening.
 3. The photolithographic mask according to claim 2, wherein said first auxiliary opening has a cross section with a width less than 0.3 λ/NA, where λ designates a wavelength of the radiation, and NA designates a numerical aperture.
 4. The photolithographic mask according to claim 2, wherein said primary opening is semi-isolated and said first auxiliary opening is associated with said primary opening.
 5. The photolithographic mask according to claim 2, wherein: said primary opening is isolated; and said first and second auxiliary openings are associated with said primary opening.
 6. The photolithographic mask according to claim 2, wherein said primary opening and said further primary opening produce a first and a second opening on the radiation-sensitive resist layer and a relative phase difference of 180° as the radiation passes therethrough.
 7. The photolithographic mask according to claim 2, wherein, said first auxiliary opening and one of said further primary opening and said second auxiliary opening producing a relative phase difference of 180° when radiation passes therethrough.
 8. The photolithographic mask according to claim 2, wherein a grid-like pattern is formed by at least one of said primary openings and said first auxiliary opening.
 9. The photolithographic mask according to claim 8, wherein: said grid-like pattern defines a period; and each of said auxiliary openings is disposed at a distance from one of said primary opening and another of said auxiliary openings, said distance being between 0.3 and 0.7 times said period of said grid-like pattern.
 10. The photolithographic mask according to claim 1, wherein: said primary opening has a rectangular structure; and said first auxiliary opening: has a cross section with a width below a limiting dimension, said limiting dimension describing a minimum structure extent on the mask necessary to form a structure on a semiconductor substrate; is formed at a distance from said primary opening, said distance: being greater than a resolution limit of a projection apparatus used for exposure based on a wafer scale; and being less than a coherence length of light used in the projection apparatus; and produces a phase difference with respect to the radiation through said adjacent opening as the radiation passes therethrough.
 11. The photolithographic mask according to claim 10, wherein said phase difference shifts a phase of the radiation from 160° to 200° with respect to said primary opening.
 12. The mask according to claim 11, wherein a length of said auxiliary opening exceeds a length of said primary opening.
 13. The mask according to claim 11, wherein, formed in said layer, a further auxiliary opening is placed mirror-symmetrically in relation to said first auxiliary opening about an axis of symmetry of said primary opening.
 14. The mask according to claim 11, wherein: said layer has a further primary opening formed therein; and said first auxiliary opening includes a number of non-coherent rectangles having a smaller distance from one another than a distance of each of said non-coherent rectangles from said further primary opening.
 15. The mask according to claim 11, wherein: said layer has a further primary opening formed therein and thereby defines a number of said primary openings; said number of said primary openings forms a regular pattern; and a number of said associated auxiliary openings also forms a regular pattern.
 16. The mask according to claim 11, wherein: said auxiliary openings are associated with said primary opening; and said primary opening is surrounded by said auxiliary openings.
 17. The mask according to claim 11, wherein said primary opening is substantially square.
 18. The mask according to claim 14, wherein said primary opening is substantially square.
 19. The mask according to claim 16, wherein said primary opening is substantially square.
 20. The mask according to claim 15, wherein: said primary openings forming said pattern are in each case square; and said respectively associated auxiliary openings have lengths exceeding a resolution limit and a width falling below a limiting dimension for forming a structure on a wafer, and in each case have a same longitudinal alignment on the mask.
 21. The mask according to claim 10, wherein: said primary opening is transparent, and said first auxiliary opening is at least semi-transparent; and an environment of said primary opening and said first auxiliary opening is opaque to the light falling therethrough.
 22. The mask according to claim 21, wherein said first auxiliary opening is transparent.
 23. The mask according to claim 10, wherein a cross section with a width has an extent below a resolution limit of a projection apparatus, based on a scale of the mask.
 24. A method for producing a structure, which comprises: using a mask according to claim 10 in an optical projection apparatus; and imaging a structure from the primary opening on a wafer, the structure having a length to width ratio greater than a length to width ratio of the opening on the mask.
 25. A method of exposing a wafer in a projection apparatus, which comprises: using a mask according to claim 1; determining a length to width ratio to be achieved of a structure to be imaged on a wafer; setting a numerical aperture of a projection apparatus at least as a function: of a length and a width of the primary opening, of a length and a width of the first auxiliary opening, of a distance from the first auxiliary opening to the primary opening, and the length to width ratio to be achieved.
 26. The method according to claim 25, wherein the setting of the numerical aperture includes conducting a numerical simulation that accounts for dependencies. 